There exist multiple types of light sources currently in use for providing illumination. Such light sources are commonly referred to as lamps. Most of the lamps in use are electrically powered. One of the most common types in use is an incandescent lamp in which a filament of tungsten or other refractory material is heated by the power dissipated in the electrical resistance of the filament when an electrical current is forced through it. Much of the dissipated power is radiated as heat in the form of infrared radiation, some of the power converts to heat that leaves the lamp through thermal conduction and convection, and a relatively small portion of the power is radiated as visible light. For an incandescent lamp the power efficiency of the lamp, which is calculated as the ratio of the power radiated as visible light to the total electrical power dissipated in the lamp, is typically about 5 percent or lower.
The envelope of an incandescent lamp is capable of operating at high temperatures, and the portion of the dissipated power that is not radiated as heat or light is usually carried away almost entirely by convection. There usually is no need for an additional heat sink.
The light radiated from the filament of an incandescent lamp emerges in all directions, and any attempt to distribute the light efficiently and uniformly over a limited illuminated area in practice requires compound optics, such as a reflector in back of the envelope and either a reflector or a lens in front of it.
Another common type of lamp is a discharge lamp, in which electrical current flows through a gas. Excited by the current, the gas emits infrared, visible, and ultraviolet radiation. A fluorescent lamp is a type of discharge lamp in which much of the ultraviolet radiation is converted to visible radiation by a fluorescent coating. Other types of discharge lamps include sodium lamps, carbon arc lamps, mercury arc lamps, neon lamps, xenon lamps, plasma lamps, and metal halide lamps. Visible light is radiated with power efficiencies ranging up to the low twenty percent range. Much of the remaining power is dissipated as infrared or ultraviolet radiation, and some may be converted to heat that is carried away through thermal conduction and convection.
Discharge lamps share with incandescent lamps the ability to shed heat without the addition of a heat sink. Discharge lamps also share with incandescent lamps the need for compound optics to direct the light efficiently and uniformly over a limited illuminated area.
A newer category of light sources distinct from incandescent lamps and discharge lamps is that of solid-state light-emitting devices. Included in this category are, for example, electroluminescent devices, semiconductor lasers, and light-emitting diodes. Unlike incandescent lamps and discharge lamps, solid-state light-emitting devices suitable for illumination emit substantially all of their radiation in the form of visible light, and the amount of power emitted in the form of infrared or ultraviolet radiation is relatively insignificant. Currently, the most efficient of these solid-state light-emitting devices, the light-emitting diodes (LEDs) and the semiconductor lasers, may operate at power efficiencies as high as twenty to forty percent. The electrical power that is not converted to light is converted to heat. To be efficient and long-lasting the solid-state devices cannot operate at high temperatures. Due to the small sizes of practical high-power devices and the low temperatures at which they must operate, usually only a small fraction of the heat is removed from the devices directly through convection, and the remainder of the heat must be removed by thermal conduction through a heat sink that in turn spreads the heat and transfers the heat to the surrounding air by way of convection over a large surface area.
Also, unlike incandescent and discharge lamps, solid-state light-emitting devices emit light over a limited range of directions. Most, in fact, emit only into a half-space, since the devices are attached to heat sinks that would block any light emitted in other directions. This fact, coupled with the fact that solid-state light-emitting devices can be very small compared to incandescent and discharge sources, may present some unique opportunities.
There are many lighting applications in which a large, flat surface, often rectangular in shape, must be illuminated with some degree of uniformity. Examples include the illumination of billboard signs; illumination of displays, paintings, food service, etc.; illumination of walls for color or dramatic effect; and indirect lighting, in which walls or ceilings are illuminated so that they will act as non-glare sources of diffuse light. The usual practice is to use spotlights or floodlights for illumination or, in the case of indirect lighting, to hide the light source within a cove that keeps direct light from striking the eyes of viewers but allows direct and diffuse reflected light to strike a wall or ceiling. The illumination resulting from these methods is often lacking in either uniformity or efficiency. Meanwhile, methods for achieving more uniform illumination, such as the use of projection optics as in a movie projector or slide projector, are usually too expensive due to the cost of the optics. In addition, the use of projection optics often requires that the light source be inconveniently distant from the object being illuminated.